Single-phase variable reluctance motors are well known in the art. They are particularly simple to construct and they can operate with a low-cost electronic controller. The single-phase variable reluctance drive is therefore an attractive choice for use in certain cost-sensitive applications such as fans and pumps.
These brushless motors employ one or more exciting windings and a magnetic circuit to produce mechanical torque which is substantially proportional to the square of the winding ampere-turns and to the angular rate of change of the magnetic circuit inductance, which is a function of the displacement of the rotor in the motor. Typically, these motors employ a stator containing one or more windings and a rotor mounted coaxially relative to the stator, typically within the stator on bearings. Displacement of the rotor relative to the stator produces a variation of the reluctance of the magnetic circuit.
The torque produced by a variable reluctance motor is proportional to the product of the square of the winding ampere-turns and the angular rate of change of inductance as a function of rotor displacement. Accordingly, motor torque that is positive with respect to some arbitrary reference can only be developed when winding ampere-turns are sustained during an interval in which the inductance increases with rotor displacement. Conversely, negative motor torque is developed when winding ampere-turns are sustained during an interval in which the inductance decreases with rotor displacement. In order to secure continuous rotation of the variable reluctance motor, it is necessary to apply ampere-turns to the motor winding during intervals of increasing inductance and to decrease or eliminate such ampere-turns during intervals of decreasing inductance.
From the foregoing, it is apparent that the winding(s) of the variable reluctance motor must be excited from a time varying source. Furthermore, the time variations of the source must be synchronized with the mechanical rotation of the rotor so that winding current is supplied to the motor during intervals in which the inductance increases with displacement and so that such current is decreased or, preferably, eliminated during the intervals in which the inductance is decreasing with rotor displacement. When a time-invariant source of electrical energy, such as a direct current source, is used, a controller is required to produce synchronized pulsations of voltage or current. The instants at which pulses are applied to and removed from the winding(s) are determined by a rotor position transducer (RPT) which sends data describing the rotor position to the controller.
The single-phase variable reluctance motor, although simple to construct, has several drawbacks.
FIG. 1 shows a typical plot of starting torque against angular rotor displacement for a known single-phase variable reluctance motor. It will be seen that the machine only develops torque in, for example, the positive direction between point 2 and point 4. To rotate in the positive direction, it would preferably only have current supplied to its exciting coil(s) when the rotor position is between points 2 and 4 and it preferably would not have current supplied when the rotor position is between points 5 and 6. In the regions between points 1 and 2 and points 4 and 5, current flowing will produce virtually no torque. It is therefore clear that the average torque in the desired direction over a complete cycle is much less than the peak torque T.sub.max.
As FIG. 1 indicates, the average starting torque of single-phase variable reluctance motors is low. This is generally not a problem for low-torque applications such as fans, but is a problem for high-torque applications such as conveyors on assembly lines. A more significant drawback with single-phase variable reluctance motors that is clear from FIG. 1 is that the rotor may assume a position at rest at which little or no torque is developed in the desired direction and hence from which it is impossible to start the motor in the desired direction. This corresponds to any position in the regions defined by points 1-2 and points 4-6.
A solution to this problem has been to use small permanent magnets to "park" the rotor in a position at which it can generate adequate starting torque, in the desired direction e.g. point 3 in FIG. 1. Such a solution has been described, e.g. in European Patents Nos. 163328 and 601818. These patents disclose the use of one or more small permanent magnets suitably affixed within the stator structure at an appropriate angle for "parking" the rotor in a position where sufficient torque is developed to restart the motor in the desired direction. As described above, there is a critical region in which the rotor must not stop if the motor is to start again. The magnet torque from the magnetic field of the magnet in this critical region must be sufficient to overcome the friction due to the bearings or the rotor will stop in the critical area. The motor will not start if this should occur. The parking magnets ensure that the rotor does not stop in this critical region.
One disadvantage with parking magnets as used in the known systems is that the parking magnets take up physical space and require additional manufacturing steps to properly position the parking magnets. Moreover, the use of parking magnets has been heretofore difficult with variable reluctance machines having two stator poles and two rotor poles. In most known variable reluctance systems employing parking magnets, both the rotor and the stator have four poles. In such 4/4 motors, a convenient position can normally be found for the parking magnets so that the position at which the rotor is brought to rest is one from which the motor will develop good starting torque when the motor is energized. In motors having two stator poles and two rotor poles (2/2 motors) it is difficult to identify a location where the magnets can be affixed and still park the rotor in a preferred position. Known techniques generally have not allowed for the convenient use of parking magnets in 2/2 motors. This is disadvantageous in that a 2/2 pole combination is often favored for high speed operation.
The present invention is directed to overcoming these and other disadvantages of the prior art.